A cable consists of one or more wires, conductors, fibers, tubes, ropes, cords, strands, or any combination of the above, which may or may not be twisted, grouped, bunched, bundled, or otherwise assembled.
Cables are used in many applications and industries. One common use is in system control or monitoring applications. Such cables can be further classified into subcategories, including:
Instrumentation Cable: Designed for use in applications requiring critical signal transmission for measurement and systems monitoring of temperature, pressure, velocity, level, etc. Can include both analog signals and digital signals. This also includes specialty cables such as thermocouple extensions cables.
Control Cable: Cable primarily used for the remote control or operation of a device.
Power Cable: Designed to distribute power from a primary power source to various types of equipment.
Communication Cable: Consisting of such cables as telephone and telegraph cables, wiring for fire and burglar alarms, and similar central-station systems, these cables are used primarily for the transfer of information for purposes of control and/or monitoring over a network of devices. These cables often use fibers or specially designed electrical conductors to ensure fast and accurate information transfer. Communication cables are capable of transmitting power as well as both sending and receiving signals from connected devices, making them very versatile.
In many instances, cables themselves are not resilient enough to withstand the environment in which they are needed. In such cases, armors are added to the cable to provide additional protection from hazards such as falling objects, explosive gasses, or corrosive materials. These cables are referred to as armored cables.
An armored cable is a cable enclosed or wrapped in a protective outer sheath or covering. Armor typically protects the cable from moisture, chemical exposure, gaseous ingression, and mechanical damage. Armored cables may be used in electrical applications and designated as MC-HL (Metal-Clad, Hazardous Location) by Underwriters' Laboratories (UL).
To determine if a cable is MC-HL rated, UL follows the recommendations of the National Electric Code (NEC). NEC organizes armored cables by applications, grouped by classes and divisions or zones.
There are three classifications for hazardous locations—Class I, Class II, and Class III. A location is considered a Class I location if there is a presence of flammable gases or vapors in the air, such as natural gas or gasoline vapor. As illustrated in Table 1, there are 5 subclassifications of Class I locations—two Divisions and three Zones.
The most widely used subclassification system in the United States consists of two divisions—Division 1 and Division 2—and was developed by the NEC. A location classified as a Division 1 contains a high enough concentration of gasses or vapor to create an explosive atmosphere during normal operating conditions. A Division 2 location is less likely to see such an atmosphere.
Class I locations may also be classified by one of three types of zones—Zone 0, Zone 1, and Zone 2. This is an NEC-adopted classification system originally developed by the International Electrotechnical Commission (IEC). While it is less widely used in the United States, this system is very common worldwide. Zone 0 locations are similar to Division 1 locations, and are exposed to ignitable atmospheres continuously or for long periods of time. Zone 1 locations are exposed to the same conditions, however, the exposure is intermittent. Zone 0 locations require additional installation standards to be met, and require all systems to be intrinsically safe.
MC-HL armored cables are considered to be suitable for use in Class I, Division 1 and/or Zone 1 locations, and may even be installed in Class I, Zone 0 locations, so long as they are installed accordingly.
TABLE 1Hazardous Location ClassificationsClassDivisionZoneClass I:Division 1:Zone 0:Flammable Gases,Where ignitable concentrationsWhere ignitable concentrationsVapors, or Liquidsof flammable gases, vapors orof flammable gases, vapors orliquids can exist all or some ofliquids can exist all of the time orthe time under normal operatingfor long periods at a time underconditions.normal operating conditions.Zone 1:Where ignitable concentrationsof flammable gases, vapors orliquids can exist some of the timeunder normal operatingconditions.Division 2:Zone 2:Where ignitable concentrationsWhere ignitable concentrationsof flammable gases, vapors orof flammable gases, vapors orliquids are not likely to existliquids are not likely to existunder normal operatingunder normal operatingconditions.conditions.
The UL 2225 standard explains the design and performance requirements of MC-HL cables. One such example of these cable performance requirements is the UL 2225 Mechanical Impact test. During this test, the cable is subjected to the impact from a twenty-five pound weight, dropped from an elevation of one foot above the cable. A flat, metal edge of the weight impacts the cable, which is resting on a rounded metal base. During the test, the cable is connected to an electrical power source, and energized. If a short occurs, the test is considered a failure. UL 2225 requires that eight out of a total of ten tests have passing results in order for a cable to meet the standards of resistance to mechanical impact for MC-HL cables.
The most common type of cable armor used in Class I, Division 1 locations is continuously-welded and corrugated aluminum armor. Manufacturers typically build these cables by wrapping or forming an aluminum strip around a cable before applying a continuous weld to join the aluminum strip longitudinally and applying corrugations by the use of a mechanical rotating head.
UL 2225 includes many performance requirements for MC-HL cables; however, the requirement to pass the UL 2225 Mechanical Impact Test is usually the test that dictates the armor design. Aluminum strips with a thickness of 0.025″ or greater is the industry standard when making MC-HL designated cables. Other properties of the aluminum armor, such as the depth and pitch of the corrugations, affect test performance as well. Due to the thickness of the aluminum and the forming limitations, the existing armors inherently have limited flexibility and higher flexural rigidity, limited bending radii, and low bending fatigue resistance.
Furthermore, while many manufacturers produce armored cables capable of bending, and market them as flexible cable armor, none of these armored cables are rated for use in Class I, Division 1 hazardous locations. Examples of the various categories of flexible cable armors include:
Metal Braided Armor: Strands of metal (typically copper, plated copper, or stainless steel) are woven to form a mechanical covering or “braid” over the cable. Since braided armors are not hermetically sealed and offer little impact resistance, they are not considered compatible for use in Class I, Division 1 hazardous locations.
Interlocked Armor: In this armor, aluminum strips are wrapped around a cable. The edges of these small aluminum strips are locked to form a flexible tube. This armor does provide decent impact resistance, however, since the armor is not hermetically sealed, it is not considered compatible for use in Class I, Division 1 hazardous locations.
Served Wire Armor (SWA): This armor consists of several metal wires (typically galvanized steel or aluminum) which are wrapped, or “served”, around the cable to provide protection. As in the examples above, the served wire armor does not provide a sealed environment and offers only slight impact resistance. Therefore, it is not considered compatible for use in Class I, Division 1 hazardous locations.
Under certain conditions, the control rooms and/or device installations (such as those in the oil and gas industry) are required by NEC codes to use Class I, Division 1 rated cables. However, users often experience difficulties in handling these armored cables, especially in tight spaces and/or extreme cold weather. This is often due to the effort required to install, move, repair, or replace these armored cables. The upstream oil drilling operations in the Alaskan Arctic slope region is a prime example of such a location or operation. Especially in severely cold weather, the installation or repair personnel need to complete the task in the shortest amount of time, and the cable needs to be resilient enough to withstand rough handling in such environments. This is difficult with current MC-HL armored cable designs.
Likewise, the training radius on currently available products requires more working space for installation, especially at cable terminations. Management of cable loops and extra clearance requirements has become a necessary evil for such installations. While most electrical system designs factor in these spaces and constraints, it is highly desirable in the industry to have cables that are more flexible and have tighter training radii.
A need exists for a flexible armored cable that addresses at least some of the above issues.